1. Field of the Invention
The present invention relates to a four-wheel drive vehicle, such as a tractor or the like, having a power transmission system for a second pair of driving wheels (front wheels) branching from a power transmission system for a first pair of driving wheels (rear wheels) equipped with a braking unit.
2. Related Art
As compared to a two-wheel drive system, a four-wheel drive system easily creates a strong propelling force so as to be effective in climbing a hill or working in a paddy field.
However, driving the front wheels on a low-resistant flat road, such as an asphalt way or the like, wastes power. Furthermore, inasmuch as the peripheral speed of the front wheels is set slightly higher than that of the rear wheels, in order to minimize the cornering radius of the vehicle, the problem arises that the grip of the rear wheels causes the front wheels to slip and to wear during usual travel on the road.
Therefore, a typical operator on a flat road shuts-off power transmission to the front wheels, so as to drive only the rear wheels. In other words, the operator causes the vehicle to travel in the two-wheel drive mode during usual travel on a road.
When the rear wheels are braked in the four-wheel drive mode, the braking force to the rear wheels extends to the front wheels such that frictional resistance is generated between all four wheels and the road. However, when the rear wheels are braked in the two-wheel drive mode, frictional resistance is only generated between the rear wheels and the road, such that the problem arises that the braking distance becomes longer than that for a four-wheel drive vehicle.